1. Related Applications
The present application claims priority to Korean Application No. 2001-57070, filed Sep. 17, 2001, entitled xe2x80x9cOptical Fingerprint Acquisition Apparatusxe2x80x9d by Jong Ik Lee, Sung Hyu Shin and Dong Won Lee and incorporate that application by reference.
2. Field of the Invention
The present invention relates to an optical fingerprint acquisition apparatus, and in particular, to an optical fingerprint acquisition apparatus capable of reducing an image distortion and a size of an optical system by deforming the shape of a prism.
3. Description of the Related Art
A fingerprint acquisition apparatus is an apparatus for acquiring fingerprints applicable to a fingerprint recognizer, which is employed in personnel airlocks, safe locking devices, access control, time attendance, computer access control, etc., by comparing an acquired fingerprint with a user""s pre-registered fingerprint. The fingerprint acquisition apparatus is roughly classified into an optical type and a non-optical type.
An optical fingerprint acquisition apparatus is an apparatus for irradiating light onto a fingerprint laid on a prism for forming an image, and reading a fingerprint image formed on an image sensor after being reflected in accordance with the shape of the valleys or ridges of the fingerprint so as to be compared with a pre-stored fingerprint. FIG. 1 shows a mechanism of a typical optical fingerprint acquisition apparatus.
The fingerprint acquisition apparatus in FIG. 1 represents a xe2x80x9cscattering typexe2x80x9d fingerprint acquisition apparatus, which comprises an image forming prism 1, a light source 3, a condensing lens 4, and an image sensor 5. The light is incident from the light source 3 to a fingerprint acquisition window 2 of the image forming prism 1 at a perpendicular angle or an angle much less than a critical angle. Therefore, the light passes through the fingerprint acquisition window 2 without reaching the image sensor at the ridges of the fingerprint that do not reach the fingerprint acquisition window 2, while the incident light is reflected and scattered from the valleys of the fingerprint. The scattered light is incident to the condensing lens 4 and detected by the image sensor 5. Accordingly, the amount of light incident from the valleys of a fingerprint to the image sensor 5 is discriminated from the amount of light incident from the ridges of a fingerprint to the image sensor 5. As a consequence, the image sensor 5 outputs electric signals of different levels in accordance with the pattern of the fingerprint. An image processor (not shown in the drawing) defines and processes an output value of the image sensor 5 by a digital signal so as to recognize patterns of the fingerprint.
Meanwhile, there has been a strong demand for reducing the size of a fingerprint acquisition apparatus in conformity with the recent trend of minimizing sizes of products. In particular, the demand is focused on reducing the size of each component as well as of an optical path. However, since it is impossible to reduce the size of human fingers and fingerprints, there is a limit to reduce the size of each component. Moreover, reducing the length of an optical path to exceed a necessary extent results in serious deterioration of the quality of a fingerprint image.
Japanese Laid-Open Patent Publication No. Hei 4-24881 (Jan. 28, 1992) discloses a method of changing the optical path to diverse directions within an image forming prism. Japanese Laid-Open Patent Publication No. Hei 2-176984 (Jul. 10, 1990) discloses a method for changing an optical path in diverse manners inside of an image forming prism and a method for correcting an image projected from the image forming prism by using a correction lens and a correction prism.
Thus, ongoing researches are being made to seek a method for correcting an image distortion in addition to a method for forming and projecting a fingerprint image. The distance (optical path) between an image sensor and a fingerprint is variable depending on a position of the fingerprint on the prism of an optical fingerprint acquisition apparatus. Therefore, the fingerprint image is distorted from its actual image at a position where an ultimate image is formed and phased open. The distortion often appears in a trapezoidal shape.
To minimize such an image distortion, distortion and astigmatism of an image are corrected by combining a condensing lens for condensing a fingerprint image projected from a prism with a correction lens or a correction prism. In that case, however, the optical system becomes larger. In other words, size of the fingerprint acquisition apparatus cannot be reduced because the condensing lens, image correction prism, path changing mirror, etc. are discretely installed. This ultimately results in an increase of the manufacturing cost and a decrease of the productivity.
A desirable solution to reduce image distortion while maintaining size of the optical system to be small is to unify the distance (length of the optical path) from any position of the fingerprint to the image sensor.
It is, therefore, an object of the present invention to provide an optical fingerprint acquisition apparatus employing a prism and a reflecting mirror that can unify the distance (length of the optical path) from any position of a fingerprint to an image sensor to minimize the optical path difference without relying on a lens or a prism for image correction.
To achieve the above and other objects of the present invention, there is provided a fingerprint acquisition apparatus for acquiring a fingerprint image by means of light scattered from valleys and ridges of a fingerprint, the apparatus comprising a prism and a reflecting mirror.
The prism comprises: a fingerprint contacting surface to be touched by a fingerprint of a person; a totally reflecting surface facing the fingerprint contacting surface for totally and inwardly reflecting the light scattered from the fingerprint in contact with the fingerprint contacting surface; a primarily projecting/re-incident surface linking the fingerprint contacting surface to the totally reflecting surface for primarily projecting the light totally reflected from the totally reflecting surface so as to be re-incident from outside; and an ultimately projecting surface facing the primarily projecting/re-incident surface for ultimately projecting the light re-incident to the primarily projecting/re-incident surface toward outside.
The reflecting mirror is installed outside adjacent to the primarily projecting/re-incident surface of the prism so as to correct the optical path difference by reflecting the light projected from the primarily projecting/re-incident surface so as to be re-incident to the primarily projecting/re-incident surface.
The prism is preferably shaped rectangular, and each surface thereof preferably has the following relation.
(Length of the Ultimately Projecting Surface) less than (Length of the Primarily Projecting/Re-Incident Surface) less than (Length of the Fingerprint Contacting Surface) less than (Length of the Totally Reflecting Surface)